Washington: Alarmed over security of weapons-grade plutonium in Iran's secretive Bushehr nuclear plant, US has significantly stepped up spying operations over the Persian Gulf country.

The Americans are increasing the surveillance of Iran's southwestern coast by deploying Pentagon's fleet of drones operating over the Persian Gulf and the electronic eye in the sky has intercepted visual images and audio communication from the reactor complex, The Wall Street Journal reported.

US drones are intercepting cellphones, electronic communications and other signals from the reactor, the paper said quoting American intelligence sources.

Surveillance at Bushehr was stepped up especially after two fuel rods from the plant were removed in October, just two months from the time it became operational, officials said.

Nuclear experts, however, at the moment are more concerned over the safety of the plant than about possibility of Iran using it to develop nuclear weapons. Tehran has been complaining that US was spying on Bushehr and had at times even sent fighter jets to pursue the unmanned craft, firing at them but missing.

Tehran formally protested the Pentagon's spying activities in a November 19 letter to United Nations Secretary General Ban Ki-moon. The complaint charged that the US has violated Iranian airspace with its drone flights. But US officials maintain that surveillance is conducted off the country's shoreline, in line with international law.

The extent of the US surveillance activity the limits of US knowledge about Iran and its military and scientific bureaucracy points to anxious efforts by Washington to increase its understanding as an international confrontation looms over Tehran's nuclear program.

The 1000-megawatt Bushehr reactor is viewed by the US as a lesser proliferation and security threat than growing number of uranium richment facilities in Iran. Both the reactor and the enrichment plants produce fissile materials that can be used to develop nuclear weapons. But Russia's State Atomic Power Corp that built Bushehr under an agreement stipulates all spent fuel would be returned to Russia and stored.

So the International Atomic Energy Agency was surprised on October 15 when Iran notified the UN watchdog that it was discharging all of the nuclear fuel from Bushehr and storing it in a cooling pond at the site, according to Vienna-based diplomats briefed on the correspondence.

Independent nuclear experts estimate that this discharged fuel was made up of between 22 and 220 pounds of weapons-grade plutonium, enough to produce as many as 24 nuclear bombs, if reprocessed further.